Warnings are quite different from errors. They are just pointing out potential issues—in a rather dumb machine way.

Often, the situation is something like this: you have, for example, a body background color of black. Then your content area has a background color of white. And then, in your content area, your text is coloured black. The validation is seeing that your text is black, and that the body background is black, and saying—hey, there's a potential problem there. Well, duh. Anyhow, you can keep the validation happy by adding a background color of white to the text ... if you can be bothered. But it's not a real problem IMHO.